There is a sensing method for use with a capacitive touch panel. This method uses each of a plurality of sensor electrodes patterned with ITO (Indium Tin Oxide) or other transparent electrodes as an independent sensor to capture and quantify capacitance changes. This method may be called a self-capacitance method or a single sensor method. Here, this method is referred to as a first method.
There is another sensing method for use with a capacitive touch panel. The functionality of this method is divided into two categories for quantification purposes: driving side and sensing side. The driving side charges and discharges capacitance generated between electrodes for driving purposes. The sensing side measures the resulting capacitance changes. This method may be called a mutual capacitance method. Here, this method is referred to as a second method.
When a multi-touch capability for simultaneously detecting two or more touch points is to be implemented for use with a touch panel based on the first method, the problem of ghost points arises so that detected coordinates do not always agree with actually touched points.
A capacitive touch panel capable of making multi-point entries is described, for instance, in Japanese Patent Application Publication JP-P2009-9249A. This touch panel is configured so that a plurality of two-dimensional capacitive sensors are positioned in close proximity to and parallel to each other.
A ghost phenomenon that may occur during the use of the first method can be avoided by determining which of two touch points was touched earlier, for instance, by using, instead of a touch panel control IC, an arithmetic processing unit that performs firmware-based computations or a control IC with a built-in microcomputer capable of performing arithmetic processing operations, and then eliminating ghost coordinates (erroneously detected coordinates) in accordance with the coordinates of the earlier-touched point.
However, the method of eliminating erroneously detected coordinates after determining which of two touch points was touched earlier successfully avoids a ghost phenomenon only when the time difference between two touches is longer than a scanning period for one sequence. Further, if any process needs to be performed by external firmware, another problem occurs to decrease the speed of processing and impose a load on an external device.
Furthermore, the touch panel described in JP-P2009-9249A accepts simultaneous multi-point entries in a limited area only.
Meanwhile, the second method makes it possible to avoid a ghost phenomenon that may arise from multi-touches. However, it is necessary to assign a control IC output to either the driving side or the sensing side. This causes a problem where the size and shape of applicable touch panels are limited or the scan rate is lower than when the first method is used.
FIG. 23 shows an example of an electrode pattern for a touch panel based on the first method. The example shown in FIG. 23 represents a pattern in which six electrodes are arranged in the x direction and five electrodes are arranged in the y direction. FIG. 24 is a diagram illustrating a capacitance measurement sequence of the touch panel shown in FIG. 23, which is based on the first method. As shown in FIG. 24, the first method, which measures the capacitance that is generated by a panel-mounted electrode and a finger, performs a line-sequential scan on all sensor terminals. FIG. 24 relates to a touch panel having an electrode pattern in which six electrodes are arranged in the x direction and five electrodes are arranged in the y direction, and indicates that the time required for one sequence is 11 T when the measurement time of each sensor is T.
Meanwhile, FIG. 25 shows an example of an electrode pattern for a touch panel based on the second method. The example shown in FIG. 25 represents a case where there are driving side terminals D1 to D6 and sensing side terminals S1 to S5. FIG. 26 is a diagram illustrating a capacitance measurement sequence of the touch panel shown in FIG. 25, which is based on the second method. As shown in FIG. 25, the second method, which measures the capacitance that is generated between two or more electrodes in the panel, performs a line-sequential scan on the sensing side with respect to a line-sequential drive of the driving side. FIG. 26 relates to a touch panel having driving side terminals D1 to D6 and sensing side terminals S1 to S5, and indicates that the time required for one sequence is 30 T. As described above, the second method requires a time of 30 T per sequence in marked contrast to the first method, which requires a time of as short as 11 T. It means that the use of the second method increases the response time.